Not local lookup

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.my-component.my-subcomponent.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.my-subcomponent.js

import Ember from 'ember';

export default Ember.Component.extend({
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component}}
<br>
<br>

templates.components.my-component.hbs

{{my-subcomponent}}

templates.components.my-component.my-subcomponent.hbs

Local subcomponent

templates.components.my-subcomponent.hbs

Root Subcomponent

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.0",
    "ember-template-compiler": "3.4.0"
  },
  "addons": {
  }
}